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> Aside from doing it the raw way, I also want to try using a 6845 I ordered
> last year to generate video. I think I've wrapped my head around how it
> works fairly well, but after looking at CGA register values, I found
> myself confused. It seems that if you're going to run
> in either 320x200 or 640x200 graphics modes, you don't set the number of
> vertically displayed rows to 200 as one might think, but 100 instead. I
> don't understand this, or how the screen would then be displayed properly,
> since it seemingly is only doing half the screen. As far as I know, CGA
> doesn't ever interlace the actual screen, despite the graphics memory
> being laid out interlaced (which I'm sure is related to this somehow). If
> anyone can shed any light on this for
> me, I'd sure appreciate it!
Well, for one thing, CGA isn't NTSC. Here's an NTSC thing I found by google (hint, hint):
formatting link
And it's interlaced. With a hot enough video amp, you could, theoretically, run 640X480, since in NTSC there are 525 lines and almost 700 pixels across, for the standard aspect ratio; but you might have to fiddle with the blanking interval(s) and stuff to actually fit all of the pixels onto the screen.
To emulate smaller CGA screens, you'd have to interpolate in S/W.
I made a B&W "TV Typewriter" some decades ago, and I don't even remember which sync generator I used, but 6845 rings a bell. But, it was just a text console; I think I managed 80 characters by 43 or 50 lines, interlaced and all that. I even pipelined the character video. :-)
Good Luck! Rich